This invention generally relates to the field of interferometry and, in particular, to apparatus and method(s) for the high accuracy measurement of aspherical optical surfaces and wavefronts using the interference of light.
A single aspherical optical surface can be used to replace several conventional spherical or plano optical surfaces or elements in an optical system to improve the system""s optical performance by reducing optical aberrations and increasing optical transmission, for example. In lithography tools where use is made of shorter wavelengths to make integrated circuits, the limited choice of materials from which refractive optical elements can be made necessitates the use of refractive and reflective aspherical optical surfaces to achieve the ever increasing performance demands for this application. Surface errors for optical surfaces needed for extreme ultraviolet (EUV) lithography tools operating at wavelengths of 13.6 nm, for example, must be less than 0.1 nm for reflective aspherical surfaces. Not only are the tolerances for the measurement accuracy increasing, but the magnitude of the aspherical departure from a reference sphere is increasing, for example, to nearly 1000 micrometers for some applications. As the integrated circuit line widths shrink, the size (diameter) of the optical elements is increasing to nearly 500 millimeters.
The measurement of aspherical surfaces and wavefronts has been very difficult because of the large departure from a best-fit reference sphere thereby producing an interferogram with many fringes that are very closely spaced. Prior art techniques have used some sort of aspherical null to mitigate this problem. Also, the Nyquist condition which requires at least two pixels per fringe, imposes a severe limit on what can be measured with available 2-D cameras. Since fringe spatial density is proportional to the slope of the aspherical surface, or wavefront, even weakly aspherical surfaces and wavefronts will violate the Nyquist condition. Typically, prior art systems are limited to surfaces with no more than 10-20 waves of aspherical departure.
In addition, for many of the prior art techniques, the wavefront difference measured in the interferogram is not simply the difference between the test and reference wavefronts. An aspherical measurement system requires that the entire interferometric measurement system, including all of the interferometer""s optics, be ray traced for the aspherical surface under test. This requirement understandably complicates the calibration of the measurement system and also reduces the accuracy of the measurements.
While high accuracy aspherical optical surfaces have been difficult and expensive to make, thereby limiting their use, there have been improvements in the fabrication of aspherical surfaces which will make them more prevalent. With fabrication improvements in, for example, magneto-rheological finishing, ion figuring, and computer controlled polishing, there is a concomitant need for improved aspherical measurement methods and apparatus to provide the error maps for these manufacturing methods to enable them to produce the aspherical optical surfaces needed to meet the requirements of integrated circuit lithography tools such a steppers and scanners operating at wavelengths of 193 nm, 157 nm, and 13.6 nm, for example.
There are many methods and apparatus in the prior art for measuring aspherical optical surfaces, for example: 1. Contacting and non-contacting stylus based profilers; 2. Contacting and non-contacting stylus based coordinate measuring machines; 3. Spherical wavefront interferometers; 4. Lateral and radial shearing interferometers; 5. Interferometers with null lenses in the measurement path; 6. Scanning spherical wave interferometers; 7. Scanning white light interferometers; 8. Sub-aperture stitching interferometers; 9. Interferometers using computer generated holograms-CGHs; 10. Point diffraction interferometers-PDIs; 11. Longer wavelength interferometry; and 12. Two wavelength interferometry. While these techniques have utility for many applications, they are limited in their operational capabilities or precision compared with those needed for today""s evolving lithography applications.
Contacting and non-contacting stylus based profilers mechanically scan the aspherical surface under test and, therefore, are slow because they measure only a few data points at a time. Slow techniques are very susceptible to measurement errors due to temperature variations during the measurement. The same limitations apply to contacting and non-contacting stylus based coordinate measuring machines.
Spherical wavefront interferometers usually require the spacing between the element generating the spherical wavefront and the aspherical surface under test to be scanned thereby increasing the measurement time for the entire surface under test thus introducing another parameter which must be measured, usually by another measurement device, and means, commonly known as stitching, for connecting the data from the various zones which fit as the spacing is scanned.
Scanning white light interferometers have many of the same limitations as spherical wavefront interferometers. Lateral and radial shearing interferometers usually measure the slope of the surface under test and thereby introduce measurement errors during the reconstruction of the surface under test via integration of the slopes. This latter type of limitation applies to differential types of profiling techniques as well.
Sub-aperture stitching interferometers introduce serious measurement errors in the stitching process. Interferometers using computer generated holograms are susceptible to errors introduced by the CGH and stray Moirxc3xa9 patterns. It is also difficult to calibrate, i.e., know the calibration of the CGH. Point diffraction interferometers are a class of spherical wavefront interferometers, and therefore, have many of the same limitations, as well as poor lateral spatial resolution.
None of the prior art approaches is entirely satisfactory since each involves a trade-off that places long lead times on the design of the measurement apparatus and method, requires additional fabrication, increases the difficulty of using and calibrating the measurement apparatus, decreases the accuracy and precision, and greatly increases the cost and delivery time of the aspherical optical element
Consequently, it is a primary object of the present invention to provide a method and apparatus for the high accuracy measurement of aspherical optical surfaces and wavefronts.
It is another object of the present invention to provide methods and apparatus for accurately measuring surfaces and wavefronts with large aspherical departures and surface slopes.
Yet another object of the present invention is to provide methods and apparatus for accurately measuring aspheric surfaces and wavefronts with large diameters (clear apertures).
Still another object of the present invention is to provide interferometric methods and apparatus for accurately measuring aspheric surfaces and wavefronts with no refractive optics in the interferometer (measurement) cavity.
Yet another object of the present invention is to provide methods and apparatus for accurately measuring aspheric surfaces and wavefronts with reduced sensitivity to temperature changes.
It is yet another object of the present invention is to provide methods and apparatus for accurately measuring aspheric surfaces and wavefronts with reduced sensitivity to turbulence of the gas in the interferometer (measurement) cavity.
Yet another object of the present invention is to provide methods and apparatus for accurately measuring aspheric surfaces and wavefronts with high speed.
Still another object of the present invention is to provide methods and apparatus for accurately measuring aspheric surfaces and wavefronts while meeting the Nyquist condition for the most challenging applications
And another object of the present invention is to provide high spatial data density methods and apparatus for accurately measuring aspheric surfaces and wavefronts.
It is another object of the present invention is to provide methods and apparatus for accurately measuring aspheric surfaces while having a relaxed tolerance to which the position on the aspherical surface from which the measurement data is gathered.
Yet another object of the present invention is to provide methods and apparatus for accurately measuring aspheric surfaces and wavefronts while having relaxed tolerance for ray tracing the interferometer""s optical system.
Still another object of the present invention is to provide methods and apparatus for accurately measuring aspheric surfaces and wavefronts while providing an error map used in the production of aspherical optical surfaces and their assembly into lens systems.
Still another object of the present invention is to provide methods and apparatus for measuring aspheric wavefronts.
Yet another object of the present invention is to provide methods and apparatus for accurately measuring aspheric surfaces in the volume production of aspherical optical surfaces and their assembly into lens systems.
Other objects of the invention will in part be obvious and will in part appear hereinafter when the following detailed description is read in connection with the accompanying drawings.
This invention relates to interferometric methodologies for the accurate measurement of aspherical surfaces and wavefronts. Method(s) and apparatus of the invention are provided for measuring aspherical optical surfaces and wavefronts that may be of large diameter and include substantial aspheric departures. The apparatus of the invention in one aspect comprises an interferometer, preferably a Fizeau interferometer, having an optical system which contains an aspherical reference surface which is illuminated by an incident aspherical wavefront generated by either refractive or diffractive optics located upstream of it. The incident aspherical wavefront is normal to the aspherical reference surface. The aspherical reference surface acts as both a beamsplitter and a reference surface for the interferometer. The aspherical reference surface reflects a portion of the incident aspherical wavefront into a reference wavefront and transmits a portion of the incident aspherical wavefront into an aspherical measurement wavefront. The aspherical reference surface and the aspherical surface under test are preferably separated by a small distance, d, so as to minimize environmentally induced measurement errors and noise. The aspherical measurement wavefront propagates normal to and is reflected by the aspherical surface under test, and the reflected aspherical measurement wavefront is recombined with the reference aspherical wavefront to form an interferogram which is indicative of the shape of the aspherical surface under test. The phase of the resulting interferogram is modulated, preferably by wavelength modulation, using any of the well known techniques for this purpose. In the instant invention, angular and positional alignment of the aspherical surface under test to the aspherical reference surface is done by analysis of the interferogram. The aspherical surface under test can then be mechanically aligned with coarse and fine positioning actuators which can be either under manual or computer control. Analysis of the final interferogram provides values of any residual misalignments.
In another aspect of the apparatus of the invention, aspheric wavefronts from one or more refractive elements are measured to determine the departure of their transmitted wavefronts compared with those anticipated.
In yet another aspect of the invention apparatus and methods are provided for aligning the aspheric reference surface with the aspheric wavefront illuminating it.